PREPARATION OF OIL 17 



suitable condition for the success of the experiments. 

 When I made further experiments with freshly bought oil, I 

 only obtained very defective results. A long course of 

 experimentation then proved that the fresh yellow oil is, as has 

 been said, unfit for the experiments, but that one can pre- 

 pare good material from it, however, by long warming up to 

 50-60 C. I heated small samples of unsuitable fresh oil 

 in flat watch-glasses in thin layers in a warm chamber, 

 such as we usually employ for embedding objects in paraffin, 

 and which is kept at a constant temperature of 54 C. The 

 yellow oil becomes by degrees quite colourless and of a 

 thicker consistency with continued heating. Under these 

 conditions the warming must always be continued eight to 

 ten days or longer, till the oil has attained the proper 

 consistency. Since commercial olive oil is in all cases very 

 variable in its nature, which moreover changes with the 

 age of the oil, the length of time that is necessary for 

 warming it cannot be determined once and for all, and it 

 can only be ascertained by repeated testings whether the 

 oil that is being warmed has gradually attained the right 

 consistency. Later I attempted to shorten the time of 

 warming by the employment of higher temperatures a 

 method which also works well. Although I did not deter- 

 mine the point more exactly, the result of my experience 

 was, that, as a rule, heating the oil up to about 80 C. for 

 two or three days has the same effect as when it is done at 

 50 to 60 C. for eight or ten days. 



As has been mentioned, the oil becomes considerably 

 thicker and more viscid during this process. I am further 

 of opinion that the right degree of consistency of the oil 

 is of special importance for the success of the experiments. 

 Oils that have become too thick give very good foams, 

 but they are unsuitable for the experiments to be described 

 hereafter, on the phenomena of streaming in the drops 

 of oil-foam, since the too great viscidity of the oil is with- 

 out doubt a hindrance to the streaming movements. Later 

 on I shall make a few remarks upon the special relations of 

 the froths prepared from oils that have been rendered viscid. 



Oil that has become too viscid can in general be cor- 



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